1 /* 2 * IPv6 Syncookies implementation for the Linux kernel 3 * 4 * Authors: 5 * Glenn Griffin <ggriffin.kernel@gmail.com> 6 * 7 * Based on IPv4 implementation by Andi Kleen 8 * linux/net/ipv4/syncookies.c 9 * 10 * This program is free software; you can redistribute it and/or 11 * modify it under the terms of the GNU General Public License 12 * as published by the Free Software Foundation; either version 13 * 2 of the License, or (at your option) any later version. 14 * 15 */ 16 17 #include <linux/tcp.h> 18 #include <linux/random.h> 19 #include <linux/siphash.h> 20 #include <linux/kernel.h> 21 #include <net/ipv6.h> 22 #include <net/tcp.h> 23 24 #define COOKIEBITS 24 /* Upper bits store count */ 25 #define COOKIEMASK (((__u32)1 << COOKIEBITS) - 1) 26 27 static siphash_key_t syncookie6_secret[2] __read_mostly; 28 29 /* RFC 2460, Section 8.3: 30 * [ipv6 tcp] MSS must be computed as the maximum packet size minus 60 [..] 31 * 32 * Due to IPV6_MIN_MTU=1280 the lowest possible MSS is 1220, which allows 33 * using higher values than ipv4 tcp syncookies. 34 * The other values are chosen based on ethernet (1500 and 9k MTU), plus 35 * one that accounts for common encap (PPPoe) overhead. Table must be sorted. 36 */ 37 static __u16 const msstab[] = { 38 1280 - 60, /* IPV6_MIN_MTU - 60 */ 39 1480 - 60, 40 1500 - 60, 41 9000 - 60, 42 }; 43 44 static u32 cookie_hash(const struct in6_addr *saddr, 45 const struct in6_addr *daddr, 46 __be16 sport, __be16 dport, u32 count, int c) 47 { 48 const struct { 49 struct in6_addr saddr; 50 struct in6_addr daddr; 51 u32 count; 52 __be16 sport; 53 __be16 dport; 54 } __aligned(SIPHASH_ALIGNMENT) combined = { 55 .saddr = *saddr, 56 .daddr = *daddr, 57 .count = count, 58 .sport = sport, 59 .dport = dport 60 }; 61 62 net_get_random_once(syncookie6_secret, sizeof(syncookie6_secret)); 63 return siphash(&combined, offsetofend(typeof(combined), dport), 64 &syncookie6_secret[c]); 65 } 66 67 static __u32 secure_tcp_syn_cookie(const struct in6_addr *saddr, 68 const struct in6_addr *daddr, 69 __be16 sport, __be16 dport, __u32 sseq, 70 __u32 data) 71 { 72 u32 count = tcp_cookie_time(); 73 return (cookie_hash(saddr, daddr, sport, dport, 0, 0) + 74 sseq + (count << COOKIEBITS) + 75 ((cookie_hash(saddr, daddr, sport, dport, count, 1) + data) 76 & COOKIEMASK)); 77 } 78 79 static __u32 check_tcp_syn_cookie(__u32 cookie, const struct in6_addr *saddr, 80 const struct in6_addr *daddr, __be16 sport, 81 __be16 dport, __u32 sseq) 82 { 83 __u32 diff, count = tcp_cookie_time(); 84 85 cookie -= cookie_hash(saddr, daddr, sport, dport, 0, 0) + sseq; 86 87 diff = (count - (cookie >> COOKIEBITS)) & ((__u32) -1 >> COOKIEBITS); 88 if (diff >= MAX_SYNCOOKIE_AGE) 89 return (__u32)-1; 90 91 return (cookie - 92 cookie_hash(saddr, daddr, sport, dport, count - diff, 1)) 93 & COOKIEMASK; 94 } 95 96 u32 __cookie_v6_init_sequence(const struct ipv6hdr *iph, 97 const struct tcphdr *th, __u16 *mssp) 98 { 99 int mssind; 100 const __u16 mss = *mssp; 101 102 for (mssind = ARRAY_SIZE(msstab) - 1; mssind ; mssind--) 103 if (mss >= msstab[mssind]) 104 break; 105 106 *mssp = msstab[mssind]; 107 108 return secure_tcp_syn_cookie(&iph->saddr, &iph->daddr, th->source, 109 th->dest, ntohl(th->seq), mssind); 110 } 111 EXPORT_SYMBOL_GPL(__cookie_v6_init_sequence); 112 113 __u32 cookie_v6_init_sequence(const struct sk_buff *skb, __u16 *mssp) 114 { 115 const struct ipv6hdr *iph = ipv6_hdr(skb); 116 const struct tcphdr *th = tcp_hdr(skb); 117 118 return __cookie_v6_init_sequence(iph, th, mssp); 119 } 120 121 int __cookie_v6_check(const struct ipv6hdr *iph, const struct tcphdr *th, 122 __u32 cookie) 123 { 124 __u32 seq = ntohl(th->seq) - 1; 125 __u32 mssind = check_tcp_syn_cookie(cookie, &iph->saddr, &iph->daddr, 126 th->source, th->dest, seq); 127 128 return mssind < ARRAY_SIZE(msstab) ? msstab[mssind] : 0; 129 } 130 EXPORT_SYMBOL_GPL(__cookie_v6_check); 131 132 struct sock *cookie_v6_check(struct sock *sk, struct sk_buff *skb) 133 { 134 struct tcp_options_received tcp_opt; 135 struct inet_request_sock *ireq; 136 struct tcp_request_sock *treq; 137 struct ipv6_pinfo *np = inet6_sk(sk); 138 struct tcp_sock *tp = tcp_sk(sk); 139 const struct tcphdr *th = tcp_hdr(skb); 140 __u32 cookie = ntohl(th->ack_seq) - 1; 141 struct sock *ret = sk; 142 struct request_sock *req; 143 int mss; 144 struct dst_entry *dst; 145 __u8 rcv_wscale; 146 147 if (!sock_net(sk)->ipv4.sysctl_tcp_syncookies || !th->ack || th->rst) 148 goto out; 149 150 if (tcp_synq_no_recent_overflow(sk)) 151 goto out; 152 153 mss = __cookie_v6_check(ipv6_hdr(skb), th, cookie); 154 if (mss == 0) { 155 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESFAILED); 156 goto out; 157 } 158 159 __NET_INC_STATS(sock_net(sk), LINUX_MIB_SYNCOOKIESRECV); 160 161 /* check for timestamp cookie support */ 162 memset(&tcp_opt, 0, sizeof(tcp_opt)); 163 tcp_parse_options(skb, &tcp_opt, 0, NULL); 164 165 if (!cookie_timestamp_decode(&tcp_opt)) 166 goto out; 167 168 ret = NULL; 169 req = inet_reqsk_alloc(&tcp6_request_sock_ops, sk, false); 170 if (!req) 171 goto out; 172 173 ireq = inet_rsk(req); 174 treq = tcp_rsk(req); 175 treq->tfo_listener = false; 176 177 if (security_inet_conn_request(sk, skb, req)) 178 goto out_free; 179 180 req->mss = mss; 181 ireq->ir_rmt_port = th->source; 182 ireq->ir_num = ntohs(th->dest); 183 ireq->ir_v6_rmt_addr = ipv6_hdr(skb)->saddr; 184 ireq->ir_v6_loc_addr = ipv6_hdr(skb)->daddr; 185 if (ipv6_opt_accepted(sk, skb, &TCP_SKB_CB(skb)->header.h6) || 186 np->rxopt.bits.rxinfo || np->rxopt.bits.rxoinfo || 187 np->rxopt.bits.rxhlim || np->rxopt.bits.rxohlim) { 188 atomic_inc(&skb->users); 189 ireq->pktopts = skb; 190 } 191 192 ireq->ir_iif = inet_request_bound_dev_if(sk, skb); 193 /* So that link locals have meaning */ 194 if (!sk->sk_bound_dev_if && 195 ipv6_addr_type(&ireq->ir_v6_rmt_addr) & IPV6_ADDR_LINKLOCAL) 196 ireq->ir_iif = tcp_v6_iif(skb); 197 198 ireq->ir_mark = inet_request_mark(sk, skb); 199 200 req->num_retrans = 0; 201 ireq->snd_wscale = tcp_opt.snd_wscale; 202 ireq->sack_ok = tcp_opt.sack_ok; 203 ireq->wscale_ok = tcp_opt.wscale_ok; 204 ireq->tstamp_ok = tcp_opt.saw_tstamp; 205 req->ts_recent = tcp_opt.saw_tstamp ? tcp_opt.rcv_tsval : 0; 206 treq->snt_synack.v64 = 0; 207 treq->rcv_isn = ntohl(th->seq) - 1; 208 treq->snt_isn = cookie; 209 treq->ts_off = 0; 210 211 /* 212 * We need to lookup the dst_entry to get the correct window size. 213 * This is taken from tcp_v6_syn_recv_sock. Somebody please enlighten 214 * me if there is a preferred way. 215 */ 216 { 217 struct in6_addr *final_p, final; 218 struct flowi6 fl6; 219 memset(&fl6, 0, sizeof(fl6)); 220 fl6.flowi6_proto = IPPROTO_TCP; 221 fl6.daddr = ireq->ir_v6_rmt_addr; 222 final_p = fl6_update_dst(&fl6, rcu_dereference(np->opt), &final); 223 fl6.saddr = ireq->ir_v6_loc_addr; 224 fl6.flowi6_oif = ireq->ir_iif; 225 fl6.flowi6_mark = ireq->ir_mark; 226 fl6.fl6_dport = ireq->ir_rmt_port; 227 fl6.fl6_sport = inet_sk(sk)->inet_sport; 228 fl6.flowi6_uid = sk->sk_uid; 229 security_req_classify_flow(req, flowi6_to_flowi(&fl6)); 230 231 dst = ip6_dst_lookup_flow(sk, &fl6, final_p); 232 if (IS_ERR(dst)) 233 goto out_free; 234 } 235 236 req->rsk_window_clamp = tp->window_clamp ? :dst_metric(dst, RTAX_WINDOW); 237 tcp_select_initial_window(tcp_full_space(sk), req->mss, 238 &req->rsk_rcv_wnd, &req->rsk_window_clamp, 239 ireq->wscale_ok, &rcv_wscale, 240 dst_metric(dst, RTAX_INITRWND)); 241 242 ireq->rcv_wscale = rcv_wscale; 243 ireq->ecn_ok = cookie_ecn_ok(&tcp_opt, sock_net(sk), dst); 244 245 ret = tcp_get_cookie_sock(sk, skb, req, dst); 246 out: 247 return ret; 248 out_free: 249 reqsk_free(req); 250 return NULL; 251 } 252